(1) Field of the Invention
The present invention relates to a toner supply device for supplying toner to a developing device provided in an image forming apparatus such as an electrophotographic copier, printer or the like, and in particular relates to a method against trouble occurring with a toner cartridge during image forming.
(2) Description of the Prior Art
An image forming apparatus using an electrophotographic process typically includes a photoreceptor as a rotational image bearer, and a charger unit, an exposure unit, a developing portion, a transfer unit, a fixing unit, a cleaning unit and a charge erasing unit, all being arranged around the photoreceptor. The charger unit electrifies the photoreceptor surface uniformly. The exposure unit forms an electrostatic latent image by illuminating the electrified photoreceptor surface with light in accordance with image information. The developing portion tribo-electrifies the toner by agitation and supplies the tribo-electrified toner to the electrostatic latent image formed on the photoreceptor surface to form a toner image. The transfer unit transfers the toner image to a recording medium by imparting charge of a polarity that is opposite to that of the toner to the recording medium. The fixing unit fixes the transferred toner image to the recording medium by a means of heating and pressing. The cleaning unit collects the toner which has not been transferred to the recording medium and is left over on the photoreceptor surface. The charge erasing unit erases charge from the photoreceptor drum after toner image transfer. The thus constructed image forming apparatus using electrophotography forms a desired image on a recording medium.
Since the toner used for development in the developing portion is an item to be consumed, this image forming apparatus includes a toner supply portion that supplies toner to the developing hopper, which is a receptacle provided for storing toner. Usually, the toner supply portion is to supply toner to the developing hopper from a toner cartridge that is removably mounted to the image forming apparatus. When all the toner in the toner cartridge has been supplied to the developer and used up, the operation of the image forming apparatus stops. When the empty toner cartridge is replaced with a new toner cartridge so as to supply the toner to the developing hopper, the operation of the image forming apparatus can be restarted.
Recently, copiers, printers and similar image forming apparatuses using an electrophotographic process which are designed for heavy users who use the apparatus with very high frequency and hence consume a large quantity of toner, have been developed. In such an image forming apparatus, the number of times the toner cartridge is replaced is reduced by designing one toner cartridge to be able to hold as much toner as possible. Use of a large capacity toner cartridge that holds a large amount of toner can reduce the number of replacements and hence reduce the number of times the image forming operation stops. Accordingly, it is desirable to light the burden on the users who are unfamiliar in handling the apparatus.
Further, in such heavy-use machines, in order to avoid suspension of the image forming operation when the toner cartridge is replaced, a reserve tank is provided between the toner cartridge and the developing portion so as to allow for replacement of the toner cartridge without cessation of the image forming operation by using the toner from the reserve tank.
As an image forming apparatus in which the toner cartridge is replaceable without stoppage of the image forming operation, a patent document 1 discloses a configuration of an image forming apparatus which includes a plurality of toner cartridges having toner of the same color and a toner feed pipe having a plurality of inlets connected to the multiple toner cartridges and joining at a midway point to deliver the toner from the toner cartridges to the developing hopper. In the image forming apparatus disclosed in this patent document 1 (Japanese Patent Application Laid-open Hei 8 No. 137227), one of the multiple toner cartridges is used to supply toner, and when the toner of the toner cartridge is used up, another toner cartridge is used to supply toner. Accordingly, the image forming operation will not be disrupted. In this case, the other toner cartridge(s) serves as a reserve tank.
Another patent document 2 (Japanese Patent Application Laid-open 2003-208007) discloses a color image forming apparatus in which a plurality of toner cartridges for black toner which is more consumed than the color toners, i.e., yellow, magenta and cyan, are provided, and the multiple toner cartridges for black toner are arranged side by side in a row. According to the image forming apparatus disclosed in this patent document 2, provision of multiple black toner cartridges makes it possible to prevent increase of the frequency of stoppage of the image forming operation due to the black toner being empty. Further, since the black toner cartridges are arranged abreast, it is possible to save the space for accommodating toner and also prevent toner cartridges from being mounted into wrong positions.
Though in the image forming apparatuses disclosed in patent documents 1 and 2, a plurality of toner cartridges for storing toner of the same color are provided to realize a (continuous run) system for automatically replacing toner cartridges for supplying toner, no measure against the troubles of the toner cartridges is taken into consideration. That is, there is a risk of occurrence of a trouble during usage of toner cartridges: for example, 1) a toner cartridge will not rotate due to an electrical connection failure; 2) failure from a burnout of the motor for toner supply; and 3) toner builds up forming aggregations around the toner supply port so that no toner can be supplied. In such cases, it is impossible for the above-described conventional image forming apparatuses to prevent stoppage of the image forming operation once any of the aforementioned troubles with toner cartridges takes place.